UNIQUE GENE HISTORIES. “Rather, a prokaryote’s core-genome may have one tree, while each and every component of its flex-genome will have an independent tree. Some horizontal gene transfer occurs even for genes in the core-genome, meaning that in principle every gene in a prokaryotic genome could have its own evolutionary history.”
https://standardsingenomics.biomedcentral.com/articles/10.1186/s40793-016-0180-8